class_docstr: Class for fitting the Free Water Tensor Model 
docstr: Initialize a FreeWaterTensorFit class instance.
  Since the free water tensor model is an extension of DTI, class
  instance is defined as subclass of the TensorFit from dti.py
  
  Parameters
  ----------
  model : FreeWaterTensorModel Class instance
      Class instance containing the free water tensor model for the fit
  model_params : ndarray (x, y, z, 13) or (n, 13)
      All parameters estimated from the free water tensor model.
      Parameters are ordered as follows:
          1) Three diffusion tensor's eigenvalues
          2) Three lines of the eigenvector matrix each containing the
             first, second and third coordinates of the eigenvector
          3) The volume fraction of the free water compartment
  
  References
  ----------
  .. [1] Henriques, R.N., Rokem, A., Garyfallidis, E., St-Jean, S.,
         Peterson E.T., Correia, M.M., 2017. [Re] Optimization of a free
         water elimination two-compartment model for diffusion tensor
         imaging. ReScience volume 3, issue 1, article number 2
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code: def __init__(self, model, model_params):
      """Initialize a FreeWaterTensorFit class instance.
      Since the free water tensor model is an extension of DTI, class
      instance is defined as subclass of the TensorFit from dti.py
  
      Parameters
      ----------
      model : FreeWaterTensorModel Class instance
          Class instance containing the free water tensor model for the fit
      model_params : ndarray (x, y, z, 13) or (n, 13)
          All parameters estimated from the free water tensor model.
          Parameters are ordered as follows:
              1) Three diffusion tensor's eigenvalues
              2) Three lines of the eigenvector matrix each containing the
                 first, second and third coordinates of the eigenvector
              3) The volume fraction of the free water compartment
  
      References
      ----------
      .. [1] Henriques, R.N., Rokem, A., Garyfallidis, E., St-Jean, S.,
             Peterson E.T., Correia, M.M., 2017. [Re] Optimization of a free
             water elimination two-compartment model for diffusion tensor
             imaging. ReScience volume 3, issue 1, article number 2
      """
      TensorFit.__init__(self, model, model_params)
  
imports: import warnings
  import numpy
  import scipy.optimize
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import TensorFit, design_matrix, decompose_tensor, _decompose_tensor_nan, from_lower_triangular, lower_triangular
  from dipy.reconst.dki import _positive_evals
  from dipy.reconst.vec_val_sum import vec_val_vect
  from dipy.core.ndindex import ndindex
  from dipy.core.gradients import check_multi_b
  from dipy.reconst.multi_voxel import multi_voxel_fit
label_desc: Use image node_id 1 to create a new FreeWaterTensorFit object from inherited base classes: TensorFit with example: obj = FreeWaterTensorFit(model, model_params)